In a medium bowl, whisk the peanut butter and lime juice until smooth. Add the brown sugar, vegetable oil and some salt and whisk until combined. Add enough water to thin the sauce and give it a dressing-like consistency, about 2 to 3 tablespoons. Place the radishes, greens, mangoes, jicama and cilantro in a large bowl and toss together. Drizzle with the dressing, toss using tongs to coat and serve.